Flat panel displays are commonly used for active matrix displays such as computer and television monitors, personal digital assistants (PDAs), cell phones and the like. Liquid crystal displays (LCD), one type of flat panel displays, generally comprise two plates, which could be glass or plastic, having a layer of liquid crystal material sandwiched therebetween. At least one of the plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive material film from the power supply changes the orientation of the liquid crystal material, creating patterns such as text or graphics that may be seen on the display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).
Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a substrate such as those utilized to fabricate flat panels. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains the substrate. The precursor gas is typically directed through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) into a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of a substrate that is positioned on a temperature controlled substrate support.
During deposition, substrate support needs to be properly RF grounded to ensure no voltage drop across the substrate support to affect deposition uniformity. Ineffective RF grounding allows plasma to travel to the side and below the substrate support to make unwanted deposition in those areas, which makes chamber cleaning more difficult and time-consuming. Some systems employ a low impedance strap to couple the substrate support to the chamber body to facilitate RF grounding of the substrate support.
FIG. 1 (Prior Art) is a simplified perspective, cutaway view of an exemplary conventional processing chamber 100 having a plurality of RF grounding straps 120 for electrically coupling a substrate support 140 to a bottom 134 of the chamber 100. Substrate transfer port 136 is an opening through which the substrate is transferred in and out of the process chamber. Eight straps 120 are shown in FIG. 1, two straps 120 being coupled to each edge of the substrate support 140. The substrate support 140 typically includes a plurality of lift pins 152, some of which are disposed along the edge of the substrate support 140 to lift the edges of the substrate during transfer. Each of the straps 120 includes first and second flexures 122, 124 joined by a bend 126. The straps 120 are generally aligned with the perimeter of the substrate support 140 and spaced to provide room for the lift pins 152 to extend below the substrate support 140. The substrate support 140 could move between a substrate loading position, which is near the lower end 138 of the transport port 136, and a substrate deposition position, which is typically above or near the higher end 139 of the transport port 136. The V-shaped straps 120 bend according to the substrate support position.
While this configuration has proven to be effective and reliable in some applications, it is less effective for systems that require the substrate supports to travel longer distances between the substrate loading positions and the substrate deposition positions. Longer traveling distance requires the RF grounding straps 120 to be longer, which would increase the impedance of the RF grounding straps and lower the RF grounding capability of the straps.
Therefore, there is a need for reliable low-impedance RF groundings that have shorter current return paths.